Hepatitis C virus entry depends on clathrin-mediated endocytosis

Characterization of hepatitis C virus particle subpopulations reveals multiple usage of the scavenger receptor BI for entry steps

Hepatitis C virus (HCV) particles assemble along the very low density lipoprotein pathway and are released from hepatocytes as entities varying in their degree of lipid and apolipoprotein (apo) association as well as buoyant densities. Little is known about the cell entry pathway of these different HCV particle subpopulations, which likely occurs by regulated spatiotemporal processes involving several cell surface molecules. One of these molecules is the scavenger receptor BI (SR-BI), a receptor for high density lipoprotein that can bind to the HCV glycoprotein E2. By studying the entry properties of infectious virus subpopulations differing in their buoyant densities, we show that these HCV particles utilize SR-BI in a manifold manner. First, SR-BI mediates primary attachment of HCV particles of intermediate density to cells. These initial interactions involve apolipoproteins, such as apolipoprotein E, present on the surface of HCV particles, but not the E2 glycoprotein, suggesting that lipoprotein components in the virion act as host-derived ligands for important entry factors such as SR-BI. Second, we found that in contrast to this initial attachment, SR-BI mediates entry of HCV particles independent of their buoyant density. This function of SR-BI does not depend on E2/SR-BI interaction but relies on the lipid transfer activity of SR-BI, probably by facilitating entry steps along with other HCV entry co-factors. Finally, our results underscore a third function of SR-BI governed by specific residues in hypervariable region 1 of E2 leading to enhanced cell entry and depending on SR-BI ability to bind to E2.

Direct-acting and host-targeting HCV inhibitors: current and future directions

The inclusion of NS3 protease inhibitors to the interferon-containing standard of care improved sustained viral response rates in hepatitis C virus (HCV) infected patients. However, there is still an unmet medical need as this drug regimen is poorly tolerated and lacks efficacy, especially in difficult-to-treat patients. Intense drug discovery and development efforts have focused on direct-acting antivirals (DAA) that target NS3 protease, NS5B polymerase and the NS5A protein. DAA combinations are currently assessed in clinical trials. Alternative antivirals have emerged that target host machineries co-opted by HCV. Finally, continuous and better understanding of HCV biology allows speculating on the value of novel classes of DAA required in future personalized all-oral interferon-free combination therapy and for supporting global disease eradication.

New hepatitis C virus drug discovery strategies and model systems

INTRODUCTION

Hepatitis C virus (HCV) is a major cause of liver disease worldwide and the leading indication for liver transplantation in the United States. Current treatment options are expensive, not effective in all patients and are associated with serious side effects. Although preclinical, anti-HCV drug screening is still hampered by the lack of readily infectable small animal models, the development of cell culture HCV experimental model systems has driven a promising new wave of HCV antiviral drug discovery.

AREAS COVERED

This review contains a concise overview of current HCV treatment options and limitations with a subsequent in-depth focus on the available experimental models and novel strategies that have, and continue to enable, important advances in HCV drug development.

EXPERT OPINION

With a large cohort of chronically HCV-infected patients progressively developing liver disease that puts them at risk for hepatocellular carcinoma and hepatic decompensation, there is an urgent need to develop effective therapeutics that are well tolerated and effective in all patients and against all HCV genotypes. Significant advances in HCV experimental model development have expedited drug discovery; however, additional progress is needed. Importantly, the current trends and momentum in the field suggests that we will continue to overcome critical experimental challenges to reach this end goal.

Hepatitis C virus induces epidermal growth factor receptor activation via CD81 binding for viral internalization and entry

While epidermal growth factor receptor (EGFR) has been shown to be important in the entry process for multiple viruses, including hepatitis C virus (HCV), the molecular mechanisms by which EGFR facilitates HCV entry are not well understood. Using the infectious cell culture HCV model (HCVcc), we demonstrate that the binding of HCVcc particles to human hepatocyte cells induces EGFR activation that is dependent on interactions between HCV and CD81 but not claudin 1. EGFR activation can also be induced by antibody mediated cross-linking of CD81. In addition, EGFR ligands that enhance the kinetics of HCV entry induce EGFR internalization and colocalization with CD81. While EGFR kinase inhibitors inhibit HCV infection primarily by preventing EGFR endocytosis, antibodies that block EGFR ligand binding or inhibitors of EGFR downstream signaling have no effect on HCV entry. These data demonstrate that EGFR internalization is critical for HCV entry and identify a hitherto-unknown association between CD81 and EGFR.

Treating hepatitis C: current standard of care and emerging direct-acting antiviral agents

Summary.  During the late 1990s and early 2000s, major advances were made in the treatment of patients with chronic hepatitis C virus (HCV) infection. Interferon, combination interferon plus ribavirin (RBV) and pegylated interferon plus RBV increased sustained virologic response (SVR) rates from ~5% to ~40-80%, depending on the genotype of HCV infection. Advances in molecular biology have allowed investigators to begin to understand the mechanisms of HCV infection and replication. Advances in understanding of viral kinetics have provided tools to identify patients who are most likely to attain SVR. With the advances in the science of HCV infection, the first part of the 21st century has seen the development and early introduction of a number of direct-acting antiviral (DAA) drugs. These novel medications interfere with critical steps in HCV replication and have the potential to significantly increase SVR rates. This article will review the key elements of HCV replication and evaluate the various classes of new and investigational DAA that have the potential to create a revolution in the management of patients with chronic hepatitis C.

Treating hepatitis C infection by targeting the host

More than 130 million people worldwide are chronically infected with the hepatitis C virus (HCV), which can lead to cirrhosis, liver failure, and hepatocellular carcinoma. Although recently approved HCV NS3-4A protease inhibitors significantly improve treatment response rates, current HCV treatment is still frequently limited by side effects and by the low genetic barrier to viral resistance against direct-acting antiviral agents. A complementary strategy is to target the host cellular factors that support the HCV life cycle. Several studies, including RNA interference screens, demonstrated that HCV depends on dozens, if not hundreds, of cellular proteins to complete its life cycle. A better understanding of the interactions between HCV proteins and host factors may help to identify host targets for antiviral therapy. In this review, we highlight some of the host factors that are particularly attractive targets for the treatment of HCV.

Hepatitis C virus attachment mediated by apolipoprotein E binding to cell surface heparan sulfate

Viruses are known to use virally encoded envelope proteins for cell attachment, which is the very first step of virus infection. In the present study, we have obtained substantial evidence demonstrating that hepatitis C virus (HCV) uses the cellular protein apolipoprotein E (apoE) for its attachment to cells. An apoE-specific monoclonal antibody was able to efficiently block HCV attachment to the hepatoma cell line Huh-7.5 as well as primary human hepatocytes. After HCV bound to cells, however, anti-apoE antibody was unable to inhibit virus infection. Conversely, the HCV E2-specific monoclonal antibody CBH5 did not affect HCV attachment but potently inhibited HCV entry. Similarly, small interfering RNA-mediated knockdown of the key HCV receptor/coreceptor molecules CD81, claudin-1, low-density lipoprotein receptor (LDLr), occludin, and SR-BI did not affect HCV attachment but efficiently suppressed HCV infection, suggesting their important roles in HCV infection at postattachment steps. Strikingly, removal of heparan sulfate from the cell surface by treatment with heparinase blocked HCV attachment. Likewise, substitutions of the positively charged amino acids with neutral or negatively charged residues in the receptor-binding region of apoE resulted in a reduction of apoE-mediating HCV infection. More importantly, mutations of the arginine and lysine to alanine or glutamic acid in the receptor-binding region ablated the heparin-binding activity of apoE, as determined by an in vitro heparin pulldown assay. HCV attachment could also be inhibited by a synthetic peptide derived from the apoE receptor-binding region. Collectively, these findings demonstrate that apoE mediates HCV attachment through specific interactions with cell surface heparan sulfate.

Modulation of hepatitis C virus release by the interferon-induced protein BST-2/tetherin

Hepatitis C virus is a leading cause of chronic hepatitis and liver cancer. Little information exists on the interplay between innate defense mechanisms and viral antagonists that promote viral egress. Herein, the effects of Tetherin/BST-2 on HCV release were investigated. In Huh-7.5 hepatocytes, low expression levels of BST-2 were detected. Treatment of Huh-7.5 cells with IFNα, elevated BST-2 expression levels. However, HCV could not alter the expression of IFNα-induced BST-2, nor of stably over-expressed BST-2. Significantly, over expressed BST-2 moderately blocked HCV production and release from Huh-7.5 cells. Functional analysis of BST-2, confirmed its ability to inhibit the release of HIV delta-Vpu from Huh-7.5-BST-2 cells. HIV-Vpu antagonized BST-2 activity and rescued HIV delta-Vpu release from Huh-7.5-BST-2 cells. However, vpu slightly rescued HCV release and production from Huh-7.5-BST-2. We conclude that BST-2 moderately restricts HCV production and release from Huh-7.5 hepatocytes, while the virus lacks mechanisms to counteract this restriction.

Computational identification of interplay between phosphorylation and O-β-glycosylation of human occludin as potential mechanism to impair hepatitis C virus entry

Hepatitis C virus (HCV) is one of the leading causes of liver diseases. Several host factors that facilitate the attachment and entry of HCV have been discovered, of which human occludin seems to be the most promising. Studies have shown that activity of occludin is dependent upon its phosphorylation status, and that during HCV infection deregulation of phosphorylated occludin collectively leads to a reduction in tight junction (TJ) integrity of hepatocytes and favors HCV entry. However, detailed information of the posttranslational modifications (PTMs) of occludin still remains largely unknown. In addition to phosphorylation, serine/threonine residues of several proteins are also regulated by a unique type of modification known as O-β-glycosylation and this crosstalk serves as a functional switch. To identify the O-β-glycosylation potential and how interplay between phosphorylation and O-β-glycosylation can be exploited for the inhibition of HCV entry, here we report a computational analysis of PTMs of human occludin. Several conserved phosphorylation residues and kinases that can alter the ability of occludin to regulate the integrity of TJs were identified. In addition to previously reported Tyr residues, two additional Tyr residues (Tyr29 and Tyr287) were identified as target sites of Src kinase. To our knowledge, this is the first study to report the O-β-GlcNAc potential of occludin and target sites of ERK (Ser8, Ser310, and Thr345), GSK-3 (Ser8, Ser341) and Cdk5 (Thr376). Furthermore, based on findings from this study, a potential novel interplay between phosphorylation and O-β-glycosylation at the two Yin Yang sites (Ser408 and Ser490) is also proposed.

Distinct roles in folding, CD81 receptor binding and viral entry for conserved histidine residues of hepatitis C virus glycoprotein E1 and E2

The protonation of histidine in acidic environments underpins its role in regulating the function of pH-sensitive proteins. For pH-sensitive viral fusion proteins, histidine protonation in the endosome leads to the activation of their membrane fusion function. The HCV (hepatitis C virus) glycoprotein E1–E2 heterodimer mediates membrane fusion within the endosome, but the roles of conserved histidine residues in the formation of a functional heterodimer and in sensing pH changes is unknown. We examined the functional roles of conserved histidine residues located within E1 and E2. The E1 mutations, H222A/R, H298R and H352A, disrupted E1–E2 heterodimerization and reduced virus entry. A total of five out of six histidine residues located within the E2 RBD (receptor-binding domain) were important for the E2 fold, and their substitution with arginine or alanine caused aberrant heterodimerization and/or CD81 binding. Distinct roles in E1–E2 heterodimerization and in virus entry were identified for His691 and His693 respectively within the membrane-proximal stem region. Viral entry and cell–cell fusion at neutral and low pH values were enhanced with H445R, indicating that the protonation state of His445 is a key regulator of HCV fusion. However, H445R did not overcome the block to virus entry induced by bafilomycin A1, indicating a requirement for an endosomal activation trigger in addition to acidic pH.

(-)-Epigallocatechin-3-gallate is a new inhibitor of hepatitis C virus entry

Here, we identify (-)-epigallocatechin-3-gallate (EGCG) as a new inhibitor of hepatitis C virus (HCV) entry. EGCG is a flavonoid present in green tea extract belonging to the subclass of catechins, which has many properties. Particularly, EGCG possesses antiviral activity and impairs cellular lipid metabolism. Because of close links between HCV life cycle and lipid metabolism, we postulated that EGCG may interfere with HCV infection. We demonstrate that a concentration of 50 μM of EGCG inhibits HCV infectivity by more than 90% at an early step of the viral life cycle, most likely the entry step. This inhibition was not observed with other members of the Flaviviridae family tested. The antiviral activity of EGCG on HCV entry was confirmed with pseudoparticles expressing HCV envelope glycoproteins E1 and E2 from six different genotypes. In addition, using binding assays at 4°C, we demonstrate that EGCG prevents attachment of the virus to the cell surface, probably by acting directly on the particle. We also show that EGCG has no effect on viral replication and virion secretion. By inhibiting cell-free virus transmission using agarose or neutralizing antibodies, we show that EGCG inhibits HCV cell-to-cell spread. Finally, by successive inoculation of naïve cells with supernatant of HCV-infected cells in the presence of EGCG, we observed that EGCG leads to undetectable levels of infection after four passages.

CONCLUSION

EGCG is a new, interesting anti-HCV molecule that could be used in combination with other direct-acting antivirals. Furthermore, it is a novel tool to further dissect the mechanisms of HCV entry into the hepatocyte.

Baculovirus GP64-mediated entry into mammalian cells

The baculovirus Autographa californica multiple nucleopolyhedrovirus (AcMNPV) serves as an efficient viral vector, not only for abundant gene expression in insect cells, but also for gene delivery into mammalian cells. Lentivirus vectors pseudotyped with the baculovirus envelope glycoprotein GP64 have been shown to acquire more potent gene transduction than those with vesicular stomatitis virus (VSV) envelope glycoprotein G. However, there are conflicting hypotheses about the molecular mechanisms of the entry of AcMNPV. Moreover, the mechanisms of the entry of pseudotyped viruses bearing GP64 into mammalian cells are not well characterized. Determination of the entry mechanisms of AcMNPV and the pseudotyped viruses bearing GP64 is important for future development of viral vectors that can deliver genes into mammalian cells with greater efficiency and specificity. In this study, we generated three pseudotyped VSVs, NPVpv, VSVpv, and MLVpv, bearing envelope proteins of AcMNPV, VSV, and murine leukemia virus, respectively. Depletion of membrane cholesterol by treatment with methyl-β-cyclodextrin, which removes cholesterol from cellular membranes, inhibited GP64-mediated internalization in a dose-dependent manner but did not inhibit attachment to the cell surface. Treatment of cells with inhibitors or the expression of dominant-negative mutants for dynamin- and clathrin-mediated endocytosis abrogated the internalization of AcMNPV and NPVpv into mammalian cells, whereas inhibition of caveolin-mediated endocytosis did not. Furthermore, inhibition of macropinocytosis reduced GP64-mediated internalization. These results suggest that cholesterol in the plasma membrane, dynamin- and clathrin-dependent endocytosis, and macropinocytosis play crucial roles in the entry of viruses bearing baculovirus GP64 into mammalian cells.

Exploitation of lipid components by viral and host proteins for hepatitis C virus infection

Hepatitis C virus (HCV), which is a major causative agent of blood-borne hepatitis, has chronically infected about 170 million individuals worldwide and leads to chronic infection, resulting in development of steatosis, cirrhosis, and eventually hepatocellular carcinoma. Hepatocellular carcinoma associated with HCV infection is not only caused by chronic inflammation, but also by the biological activity of HCV proteins. HCV core protein is known as a main component of the viral nucleocapsid. It cooperates with host factors and possesses biological activity causing lipid alteration, oxidative stress, and progression of cell growth, while other viral proteins also interact with host proteins including molecular chaperones, membrane-anchoring proteins, and enzymes associated with lipid metabolism to maintain the efficiency of viral replication and production. HCV core protein is localized on the surface of lipid droplets in infected cells. However, the role of lipid droplets in HCV infection has not yet been elucidated. Several groups recently reported that other viral proteins also support viral infection by regulation of lipid droplets and core localization in infected cells. Furthermore, lipid components are required for modification of host factors and the intracellular membrane to maintain or up-regulate viral replication. In this review, we summarize the current status of knowledge regarding the exploitation of lipid components by viral and host proteins in HCV infection.

Hepatitis C virus induces CD81 and claudin-1 endocytosis

Hepatitis C virus (HCV) leads to progressive liver disease and hepatocellular carcinoma. Current treatments are only partially effective, and new therapies targeting viral and host pathways are required. Virus entry into a host cell provides a conserved target for therapeutic intervention. Tetraspanin CD81, scavenger receptor class B member I, and the tight-junction proteins claudin-1 and occludin have been identified as essential entry receptors. Limited information is available on the role of receptor trafficking in HCV entry. We demonstrate here that anti-CD81 antibodies inhibit HCV infection at late times after virus internalization, suggesting a role for intracellular CD81 in HCV infection. Several tetraspanins have been reported to internalize via motifs in their C-terminal cytoplasmic domains; however, CD81 lacks such motifs, leading several laboratories to suggest a limited role for CD81 endocytosis in HCV entry. We demonstrate CD81 internalization via a clathrin- and dynamin-dependent process, independent of its cytoplasmic domain, suggesting a role for associated partner proteins in regulating CD81 trafficking. Live cell imaging demonstrates CD81 and claudin-1 coendocytosis and fusion with Rab5 expressing endosomes, supporting a role for this receptor complex in HCV internalization. Receptor-specific antibodies and HCV particles increase CD81 and claudin-1 endocytosis, supporting a model wherein HCV stimulates receptor trafficking to promote particle internalization.

New targets for antiviral therapy of chronic hepatitis C

Until recently, chronic hepatitis C caused by persistent infection with the hepatitis C virus (HCV) has been treated with a combination of pegylated interferon-alpha (PEG-IFNα) and ribavirin (RBV). This situation has changed with the development of two drugs targeting the NS3/4A protease, approved for combination therapy with PEG-IFNα/RBV for patients infected with genotype 1 viruses. Moreover, two additional viral proteins, the RNA-dependent RNA polymerase (residing in NS5B) and the NS5A protein have emerged as promising drug targets and a large number of antivirals targeting these proteins are at different stages of clinical development. Although this progress is very promising, it is not clear whether these new compounds will suffice to eradicate the virus in an infected individual, ideally by using a PEG-IFNα/RBV-free regimen, or whether additional compounds targeting other factors that promote HCV replication are required. In this respect, host cell factors have emerged as a promising alternative. They reduce the risk of development of antiviral resistance and they increase the chance for broad-spectrum activity, ideally covering all HCV genotypes. Work in the last few years has identified several host cell factors used by HCV for productive replication. These include, amongst others, cyclophilins, especially cyclophilinA (cypA), microRNA-122 (miR-122) or phosphatidylinositol-4-kinase III alpha. For instance, cypA inhibitors have shown to be effective in combination therapy with PEG-IFN/RBV in increasing the sustained viral response (SVR) rate significantly compared to PEG-IFN/RBV. This review briefly summarizes recent advances in the development of novel antivirals against HCV.

Permissiveness of human hepatoma cell lines for HCV infection

Background

Although primary and established human hepatoma cell lines have been evaluated for hepatitis C virus (HCV) infection in vitro, thus far only Huh7 cells have been found to be highly permissive for infectious HCV. Since our understanding of the HCV lifecycle would benefit from the identification of additional permissive cell lines, we assembled a panel of hepatic and non-hepatic cell lines and assessed their ability to support HCV infection. Here we show infection of the human hepatoma cell lines PLC/PRF/5 and Hep3B with cell culture-derived HCV (HCVcc), albeit to lower levels than that achieved in Huh7 cells. To better understand the reduced permissiveness of PLC and Hep3B cells for HCVcc infection, we performed studies to evaluate the ability of each cell line to support specific steps of the viral lifecycle (i.e. entry, replication, egress and spread).

Results

We found that while the early events in HCV infection (i.e. entry plus replication initiation) are cumulatively equivalent or only marginally reduced in PLC and Hep3B cells, later steps of the viral life cycle such as steady-state replication, de novo virus production and/or spread are impaired to different degrees in PLC and Hep3B cultures compared to Huh7 cell cultures. Interestingly, we also observed that interferon stimulated gene (i.e. ISG56) expression was significantly and differentially up-regulated in PLC and Hep3B cells following viral infection.

Conclusions

We conclude that the restrictions observed later during HCV infection in these cell lines could in part be attributed to HCV-induced innate signaling. Nevertheless, the identification of two new cell lines capable of supporting authentic HCVcc infection, even at reduced levels, expands the current repertoire of cell lines amendable for the study of HCV in vitro and should aid in further elucidating HCV biology and the cellular determinants that modulate HCV infection.

Development and Applications of VSV Vectors Based on Cell Tropism

Viral vectors have been available in various fields such as medical and biological research or gene therapy applications. Targeting vectors pseudotyped with distinct viral envelope proteins that influence cell tropism and transfection efficiency are useful tools not only for examining entry mechanisms or cell tropisms but also for vaccine vector development. Vesicular stomatitis virus (VSV) is an excellent candidate for development as a pseudotype vector. A recombinant VSV lacking its own envelope (G) gene has been used to produce a pseudotype or recombinant VSV possessing the envelope proteins of heterologous viruses. These viruses possess a reporter gene instead of a VSV G gene in their genome, and therefore it is easy to evaluate their infectivity in the study of viral entry, including identification of viral receptors. Furthermore, advantage can be taken of a property of the pseudotype VSV, which is competence for single-round infection, in handling many different viruses that are either difficult to amplify in cultured cells or animals or that require specialized containment facilities. Here we describe procedures for producing pseudotype or recombinant VSVs and a few of the more prominent examples from envelope viruses, such as hepatitis C virus, Japanese encephalitis virus, baculovirus, and hemorrhagic fever viruses.

Human SCARB2-mediated entry and endocytosis of EV71

Enterovirus (EV) 71 infection is known to cause hand-foot-and-mouth disease (HFMD) and in severe cases, induces neurological disorders culminating in fatality. An outbreak of EV71 in South East Asia in 1997 affected over 120,000 people and caused neurological disorders in a few individuals. The control of EV71 infection through public health interventions remains minimal and treatments are only symptomatic. Recently, human scavenger receptor class B, member 2 (SCARB2) has been reported to be a cellular receptor of EV71. We expressed human SCARB2 gene in NIH3T3 cells (3T3-SCARB2) to study the mechanisms of EV71 entry and infection. We demonstrated that human SCARB2 serves as a cellular receptor for EV71 entry. Disruption of expression of SCARB2 using siRNAs can interfere EV71 infection and subsequent inhibit the expression of viral capsid proteins in RD and 3T3-SCARB2 but not Vero cells. SiRNAs specific to clathrin or dynamin or chemical inhibitor of clathrin-mediated endocytosis were all capable of interfering with the entry of EV71 into 3T3-SCARB2 cells. On the other hand, caveolin specific siRNA or inhibitors of caveolae-mediated endocytosis had no effect, confirming that only clathrin-mediated pathway was involved in EV71 infection. Endocytosis of EV71 was also found to be pH-dependent requiring endosomal acidification and also required intact membrane cholesterol. In summary, the mechanism of EV71 entry through SCARB2 as the receptor for attachment, and its cellular entry is through a clathrin-mediated and pH-dependent endocytic pathway. This study on the receptor and endocytic mechanisms of EV71 infection is useful for the development of effective medications and prophylactic treatment against the enterovirus.

Molecular determinants and dynamics of hepatitis C virus secretion

The current model of hepatitis C virus (HCV) production involves the assembly of virions on or near the surface of lipid droplets, envelopment at the ER in association with components of VLDL synthesis, and egress via the secretory pathway. However, the cellular requirements for and a mechanistic understanding of HCV secretion are incomplete at best. We combined an RNA interference (RNAi) analysis of host factors for infectious HCV secretion with the development of live cell imaging of HCV core trafficking to gain a detailed understanding of HCV egress. RNAi studies identified multiple components of the secretory pathway, including ER to Golgi trafficking, lipid and protein kinases that regulate budding from the trans-Golgi network (TGN), VAMP1 vesicles and adaptor proteins, and the recycling endosome. Our results support a model wherein HCV is infectious upon envelopment at the ER and exits the cell via the secretory pathway. We next constructed infectious HCV with a tetracysteine (TC) tag insertion in core (TC-core) to monitor the dynamics of HCV core trafficking in association with its cellular cofactors. In order to isolate core protein movements associated with infectious HCV secretion, only trafficking events that required the essential HCV assembly factor NS2 were quantified. TC-core traffics to the cell periphery along microtubules and this movement can be inhibited by nocodazole. Sub-populations of TC-core localize to the Golgi and co-traffic with components of the recycling endosome. Silencing of the recycling endosome component Rab11a results in the accumulation of HCV core at the Golgi. The majority of dynamic core traffics in association with apolipoprotein E (ApoE) and VAMP1 vesicles. This study identifies many new host cofactors of HCV egress, while presenting dynamic studies of HCV core trafficking in infected cells.

The current model of HCV egress is that virions assemble at lipid droplets, envelope at the ER and then likely exit the hepatocyte via the secretory pathway in association with apolipoproteins. To gain a more detailed insight into infectious HCV release, we combined an RNAi analysis of host factors that are required for infectious HCV secretion with live cell imaging of HCV core trafficking. Using this approach, we identified numerous components of the secretory pathway that are both required for infectious HCV release and co-traffic with HCV core. The dynamics of HCV core trafficking, both in terms of frequency of transport, particle velocity, and the corresponding run lengths were quantified. We observe that dynamic core movements in the periphery require NS2, a viral protein required for virion assembly. Core co-traffics with multiple components of the secretory pathway, including the Golgi, recycling endosome, microtubules, VAMP1 secretory vesicles, and ApoE. This study identifies new molecular determinants of HCV secretion and describes the dynamics of their movements with HCV core in real time.

Hepatitis C virus host cell entry

The hepatitis C virus (HCV) is a major medical problem with at least 130 million infected individuals worldwide. Over the last decade multiple host factors required for HCV cell entry have been identified, but a detailed understanding of their mechanistic interplay remains elusive. Nonetheless, recent advances in defining species-specific barriers of HCV transmission have allowed the identification of a minimal set of entry factors that are required for HCV infection of rodent cells and has culminated in an animal model that recapitulates HCV entry in vivo. A detailed understanding of the viral uptake pathway is imperative to define new drug targets allowing for more effective intervention against this devastating disease.

In vitro systems for the study of hepatitis C virus infection

The study of a virus is made possible by the availability of culture systems in which the viral lifecycle can be realized. Such systems support robust virus entry, replication, assembly, and secretion of nascent virions. Furthermore, culture models provide a platform in which therapeutic interventions can be devised or monitored. Hepatitis C virus (HCV) has a restricted tropism to human and chimpanzees; thus investigations of HCV biology have been hindered for many years due to a lack of small animal models. Nevertheless, significant efforts have been directed at developing cell culture models to elucidate the viral lifecycle in vitro. HCV primarily infects liver parenchymal cells commonly known as hepatocytes. The liver is a highly specialized and complex organ and the development of in vitro systems that reflects this complexity has proven difficult. Consequently, host cell receptor molecules that potentiate HCV infection were identified over a decade after the virus was discovered. A summary of the various HCV in vitro culture models, their advantages, and disadvantages are described.

Inhibitors of alphavirus entry and replication identified with a stable Chikungunya replicon cell line and virus-based assays

Chikungunya virus (CHIKV), an alphavirus, has recently caused epidemic outbreaks and is therefore considered a re-emerging pathogen for which no effective treatment is available. In this study, a CHIKV replicon containing the virus replicase proteins together with puromycin acetyltransferase, EGFP and Renilla luciferase marker genes was constructed. The replicon was transfected into BHK cells to yield a stable cell line. A non-cytopathic phenotype was achieved by a Pro718 to Gly substitution and a five amino acid insertion within non-structural protein 2 (nsP2), obtained through selection for stable growth. Characterization of the replicon cell line by Northern blotting analysis revealed reduced levels of viral RNA synthesis. The CHIKV replicon cell line was validated for antiviral screening in 96-well format and used for a focused screen of 356 compounds (natural compounds and clinically approved drugs). The 5,7-dihydroxyflavones apigenin, chrysin, naringenin and silybin were found to suppress activities of EGFP and Rluc marker genes expressed by the CHIKV replicon. In a concomitant screen against Semliki Forest virus (SFV), their anti-alphaviral activity was confirmed and several additional inhibitors of SFV with IC₅₀ values between 0.4 and 24 µM were identified. Chlorpromazine and five other compounds with a 10H-phenothiazinyl structure were shown to inhibit SFV entry using a novel entry assay based on a temperature-sensitive SFV mutant. These compounds also reduced SFV and Sindbis virus-induced cytopathic effect and inhibited SFV virion production in virus yield experiments. Finally, antiviral effects of selected compounds were confirmed using infectious CHIKV. In summary, the presented approach for discovering alphaviral inhibitors enabled us to identify potential lead structures for the development of alphavirus entry and replication phase inhibitors as well as demonstrated the usefulness of CHIKV replicon and SFV as biosafe surrogate models for anti-CHIKV screening.

Infectious spleen and kidney necrosis virus (a fish iridovirus) enters Mandarin fish fry cells via caveola-dependent endocytosis

Infectious spleen and kidney necrosis virus (ISKNV) is the type species of the genus Megalocytivirus from the family Iridoviridae. Megalocytiviruses have been implicated in more than 50 fish species infections and currently threaten the aquaculture industry, causing great economic losses in China, Japan, and Southeast Asia. However, the cellular entry mechanisms of megalocytiviruses remain largely uncharacterized. In this study, the main internalization mechanism of ISKNV was investigated by using mandarin fish fry (MFF-1) cells. The progression of ISKNV infection is slow, and infection is not inhibited when the cells are treated with ammonium chloride (NH(4)Cl), chloroquine, sucrose, and chlorpromazine, which are inhibitors of clathrin-dependent endocytosis. The depletion of cellular cholesterol by methyl-β-cyclodextrin results in the significant inhibition of ISKNV infection; however, the infection is resumed with cholesterol replenishment. Inhibitors of caveolin-1-involved signaling events, including phorbol 12-myristate 13-acetate (PMA), genistein, and wortmannin, impair ISKNV entry into MFF-1 cells. Moreover, ISKNV entry is dependent on dynamin and the microtubule cytoskeleton. Cofraction analysis of ISKNV and caveolin-1 showed that ISKNV colocates with caveolin-1 during virus infection. These results indicate that ISKNV entry into MFF-1 cells proceeds via classical caveola-mediated endocytosis and is dependent on the microtubules that serve as tracks along which motile cavicles may move via a caveola-caveosome-endoplasmic reticulum (ER) pathway. As a fish iridovirus, ISKNV entry into MFF-1 cells is different from the clathrin-mediated endocytosis of frog virus 3 entry into mammalian cells (BHK-21) at 28°C, which has been recognized as a model for iridoviruses. Thus, our work may help further the understanding of the initial steps of iridovirus infection.

The cell biology of receptor-mediated virus entry

The cell imposes multiple barriers to virus entry. However, viruses exploit fundamental cellular processes to gain entry to cells and deliver their genetic cargo. Virus entry pathways are largely defined by the interactions between virus particles and their receptors at the cell surface. These interactions determine the mechanisms of virus attachment, uptake, intracellular trafficking, and, ultimately, penetration to the cytosol. Elucidating the complex interplay between viruses and their receptors is necessary for a full understanding of how these remarkable agents invade their cellular hosts.

Self-assembling peptide nanotubes with antiviral activity against hepatitis C virus

Hepatitis C virus (HCV) infects chronically 3% of the world population and the current therapy against this pathogen is only partially effective. With the aim of developing novel antiviral strategies against HCV, we screened a D,L-α-peptide library using an unbiased methodology based on a cell culture infection system for HCV. We found a family of highly active amphiphilic eight-residue cyclic D,L-α-peptides that specifically blocked entry of all tested HCV genotypes into target cells at a postbinding step without affecting infection by other enveloped RNA viruses. Structure-activity relationship studies indicate that antiviral activity was dependent on cyclic D,L-α-peptide self-assembly processes and that, although they possess a net neutral charge, they display a characteristic charge distribution. Our results indicate that supramolecular amphiphilic peptide structures constitute a class of highly selective HCV entry inhibitors.

Hepatitis C virus entry into the hepatocyte

Hepatitis C virus (HCV) is a small enveloped virus with a positive stranded RNA genome belonging to the Flaviviridae family. The virion has the unique ability of forming a complex with lipoproteins, which is known as the lipoviroparticle. Lipoprotein components as well as the envelope proteins, E1 and E2, play a key role in virus entry into the hepatocyte. HCV entry is a complex multistep process involving sequential interactions with several cell surface proteins. The virus relies on glycosaminoglycans and possibly the low-density lipoprotein receptors to attach to cells. Furthermore, four specific entry factors are involved in the following steps which lead to virus internalization and fusion in early endosomes. These molecules are the scavenger receptor SRB1, tetraspanin CD81 and two tight junction proteins, Claudin-1 and Occludin. Although they are essential to HCV entry, the precise role of these molecules is not completely understood. Finally, hepatocytes are highly polarized cells and which likely affects the entry process. Our current knowledge on HCV entry is summarized in this review.

The role of neutralizing antibodies in hepatitis C virus infection

Hepatitis C virus (HCV) is a blood-borne virus estimated to infect around 170 million people worldwide and is, therefore, a major disease burden. In some individuals the virus is spontaneously cleared during the acute phase of infection, whilst in others a persistent infection ensues. Of those persistently infected, severe liver diseases such as cirrhosis and primary liver cancer may develop, although many individuals remain asymptomatic. A range of factors shape the course of HCV infection, not least host genetic polymorphisms and host immunity. A number of studies have shown that neutralizing antibodies (nAb) arise during HCV infection, but that these antibodies differ in their breadth and mechanism of neutralization. Recent studies, using both mAbs and polyclonal sera, have provided an insight into neutralizing determinants and the likely protective role of antibodies during infection. This understanding has helped to shape our knowledge of the overall structure of the HCV envelope glycoproteins--the natural target for nAb. Most nAb identified to date target receptor-binding sites within the envelope glycoprotein E2. However, there is some evidence that other viral epitopes may be targets for antibody neutralization, suggesting the need to broaden the search for neutralization epitopes beyond E2. This review provides a comprehensive overview of our current understanding of the role played by nAb in HCV infection and disease outcome and explores the limitations in the study systems currently used. In addition, we briefly discuss the potential therapeutic benefits of nAb and efforts to develop nAb-based therapies.

The hepatitis C virus glycan shield and evasion of the humoral immune response

Despite the induction of effective immune responses, 80% of hepatitis C virus (HCV)-infected individuals progress from acute to chronic hepatitis. In contrast to the cellular immune response, the role of the humoral immune response in HCV clearance is still subject to debate. Indeed, HCV escapes neutralizing antibodies in chronically infected patients and reinfection has been described in human and chimpanzee. Studies of antibody-mediated HCV neutralization have long been hampered by the lack of cell-culture-derived virus and the absence of a small animal model. However, the development of surrogate models and recent progress in HCV propagation in vitro now enable robust neutralization assays to be performed. These advances are beginning to shed some light on the mechanisms of HCV neutralization. This review summarizes the current state of knowledge of the viral targets of anti-HCV-neutralizing antibodies and the mechanisms that enable HCV to evade the humoral immune response. The recent description of the HCV glycan shield that reduces the immunogenicity of envelope proteins and masks conserved neutralizing epitopes at their surface constitutes the major focus of this review.

Role of the clathrin terminal domain in regulating coated pit dynamics revealed by small molecule inhibition

Clathrin-mediated endocytosis (CME) regulates many cell physiological processes such as the internalization of growth factors and receptors, entry of pathogens, and synaptic transmission. Within the endocytic network, clathrin functions as a central organizing platform for coated pit assembly and dissociation via its terminal domain (TD). We report the design and synthesis of two compounds named pitstops that selectively block endocytic ligand association with the clathrin TD as confirmed by X-ray crystallography. Pitstop-induced inhibition of clathrin TD function acutely interferes with receptor-mediated endocytosis, entry of HIV, and synaptic vesicle recycling. Endocytosis inhibition is caused by a dramatic increase in the lifetimes of clathrin coat components, including FCHo, clathrin, and dynamin, suggesting that the clathrin TD regulates coated pit dynamics. Pitstops provide new tools to address clathrin function in cell physiology with potential applications as inhibitors of virus and pathogen entry and as modulators of cell signaling.

Hepatitis C virus (HCV) envelope glycoproteins E1 and E2 contain reduced cysteine residues essential for virus entry

The HCV envelope glycoproteins E1 and E2 contain eight and 18 highly conserved cysteine residues, respectively. Here, we examined the oxidation state of E1E2 heterodimers incorporated into retroviral pseudotyped particles (HCVpp) and investigated the significance of free sulfhydryl groups in cell culture-derived HCV (HCVcc) and HCVpp entry. Alkylation of free sulfhydryl groups on HCVcc/pp with a membrane-impermeable sulfhydryl-alkylating reagent 4-(N-maleimido)benzyl-α-trimethylammonium iodide (M135) prior to virus attachment to cells abolished infectivity in a dose-dependent manner. Labeling of HCVpp envelope proteins with EZ-Link maleimide-PEG2-biotin (maleimide-biotin) detected free thiol groups in both E1 and E2. Unlike retroviruses that employ disulfide reduction to facilitate virus entry, the infectivity of alkylated HCVcc could not be rescued by addition of exogenous reducing agents. Furthermore, the infectivity of HCVcc bound to target cells was not affected by addition of M135 indicative of a change in glycoprotein oxidation state from reduced to oxidized following virus attachment to cells. By contrast, HCVpp entry was reduced by 61% when treated with M135 immediately following attachment to cells, suggesting that the two model systems might demonstrate variations in oxidation kinetics. Glycoprotein oxidation was not altered following binding of HCVpp incorporated E1E2 to soluble heparin or recombinant CD81. These results suggest that HCV entry is dependent on the presence of free thiol groups in E1 and E2 prior to cellular attachment and reveals a new essential component of the HCV entry process.

Expression of human factors CD81, claudin-1, scavenger receptor, and occludin in mouse hepatocytes does not confer susceptibility to HCV entry

No suitable mouse model is available for studying chronic liver disease caused by hepatitis C virus (HCV). CD81, claudin-1, scavenger receptor class B type I, and occludin were recently reported to be the important factors in HCV entry into hepatocytes. We made transgenic mice (Alb-CCSO) expressing the four human proteins and examined whether HCV from a patient serum or HCV pseudoparticles (HCVpp) were capable of infecting them. HCV was not detected in the mouse serum after injecting the mice with HCV from a patient serum. We also found no indications of HCVpp entry into primary hepatocytes from Alb-CCSO mice. In addition, HCV-infectible Hep3B cells were fused with HCV-resistant primary mouse hepatocytes and the fused cells showed 35-fold lower infectivity compared to wild-type Hep3B cells, indicating that primary mouse hepatocytes have the inhibitory factor(s) in HCVpp entry. Our results suggest that the expression of the human factors does not confer susceptibility to HCV entry into the liver.

TARGETING THE NS5A PROTEIN OF HCV: AN EMERGING OPTION

Hepatitis C virus (HCV) infects more than 3% of the world's population, leading to an increased risk of cirrhosis and hepatocellular carcinoma. The current standard of care, a combination of pegylated interferon alfa and ribavirin, is poorly tolerated and often ineffective against the most prevalent genotype of the virus, genotype 1. The very recent approval of boceprevir and telaprevir, two HCV protease inhibitors, promises to significantly improve treatment options and outcomes. In addition to the viral protease NS3 and the viral polymerase NS5B, direct-acting antivirals are now in development against NS5A. A multifunctional phosphoprotein, NS5A is essential to HCV genome replication, but has no known enzymatic function. Here we report how the design of small-molecule inhibitors against NS5A has evolved from promising monomers to highly potent dimeric compounds effective against many HCV genotypes. We also highlight recent clinical data and how the inhibitors may bind to NS5A, itself capable of forming dimers.

Phospholipid scramblase 1 mediates hepatitis C virus entry into host cells

Hepatitis C virus (HCV) infects human hepatocytes through several host factors. However, other prerequisite factors for viral entry remain to be identified. Using a yeast two-hybrid screen, we found that human phospholipid scramblase 1 interacts with HCV envelope proteins E1 and E2. These physical interactions were confirmed by co-immunoprecipitation and GST pull-down assays. Knocking down the expression of PLSCR1 inhibited the entry of HCV pseudoparticles. Moreover, PLSCR1 was required for the initial attachment of HCV onto hepatoma cells, where it specifically interacted with entry factor OCLN. We show that PLSCR1 is a novel attachment factor for HCV entry.

Engineered lentiviral vectors pseudotyped with a CD4 receptor and a fusogenic protein can target cells expressing HIV-1 envelope proteins

Lentiviral vectors (LVs) derived from human immunodeficiency virus type 1 (HIV-1) are promising vehicles for gene delivery because they not only efficiently transduce both dividing and non-dividing cells, but also maintain long-term transgene expression. Development of an LV system capable of transducing cells in a cell type-specific manner can be beneficial for certain applications that rely on targeted gene delivery. Previously it was shown that an inverse fusion strategy that incorporated an HIV-1 receptor (CD4) and its co-receptor (CXCR4 or CCR5) onto vector surfaces could confer to LVs the ability to selectively deliver genes to HIV-1 envelope-expressing cells. To build upon this work, we aim to improve its relatively low transduction efficiency and circumvent its inability to target multiple tropisms of HIV-1 by a single vector. We investigated a method to create LVs co-enveloped with the HIV-1 cellular receptor CD4 and a fusogenic protein derived from the Sindbis virus glycoprotein and tested its efficiency to selectively deliver genes into cells expressing HIV-1 envelope proteins. The engineered LV system yields a higher level of transduction efficiency and a broader tropism towards cells displaying the HIV-1 envelope protein (Env) than the previously developed system. Furthermore, we demonstrated in vitro that this engineered LV can preferentially deliver suicide gene therapy to HIV-1 envelope-expressing cells. We conclude that it is potentially feasible to target LVs towards HIV-1-infected cells by functional co-incorporation of the CD4 and fusogenic proteins, and provide preliminary evidence for further investigation on a potential alternative treatment for eradicating HIV-1-infected cells that produce drug-resistant viruses after highly active antiretroviral therapy (HAART).

Mechanism of hepatitis C virus infection

About 170 million persons are infected with hepatitis C virus (HCV) around the world, and nearly 80% of infected patients develop chronic liver disease that may eventually lead to liver cirrhosis or hepatocellular carcinoma. The mechanisms underlying the life cycle of HCV in the host are still largely unknown and the efforts made by researchers have been hampered by the absence of a robust system reproducing HCV infection. Moreover, there are no effective vaccines or drugs available to defend or exclude viruses because of frequent viral mutation. In 2005, several research groups have successfully established cell culture systems for HCV, pushing the basic research on HCV to a new stage. This paper will focus on HCV genome diversity, progress in culture models, HCV life cycle, and protein function to highlight the mechanism of HCV infection.

Hepatitis C virus is primed by CD81 protein for low pH-dependent fusion

Hepatitis C virus (HCV) entry into permissive cells is a complex process that involves interactions with at least four co-factors followed by endocytosis and low pH-dependent fusion with endosomes. The precise sequence of receptor engagement and their roles in promoting HCV E1E2 glycoprotein-mediated fusion are poorly characterized. Because cell-free HCV tolerates an acidic environment, we hypothesized that binding to one or more receptors on the cell surface renders E1E2 competent to undergo low pH-induced conformational changes and promote fusion with endosomes. To test this hypothesis, we examined the effects of low pH and of the second extracellular loop (ECL2) of CD81, one of the four entry factors, on HCV infectivity. Pretreatment with an acidic buffer or with ECL2 enhanced infection through changing the E1E2 conformation, as evidenced by the altered reactivity of these proteins with conformation-specific antibodies and stable association with liposomes. However, neither of the two treatments alone permitted direct fusion with the cell plasma membrane. Sequential HCV preincubation with ECL2 and acidic buffer in the absence of target cells resulted in a marked loss of infectivity, implying that the receptor-bound HCV is primed for low pH-dependent conformational changes. Indeed, soluble receptor-pretreated HCV fused with the cell plasma membrane at low pH under conditions blocking an endocytic entry pathway. These findings suggest that CD81 primes HCV for low pH-dependent fusion early in the entry process. The simple triggering paradigm and intermediate conformations of E1E2 identified in this study could help guide future vaccine and therapeutic efforts to block HCV infection.

Hepatitis C virus entry and the tetraspanin CD81

CD81, a member of the tetraspanin integral membrane protein family, has been identified as an essential receptor for HCV (hepatitis C virus). The present review highlights recent published data on the role that CD81 plays in HCV entry, including the importance of actin-dependent lateral diffusion of CD81 within the cell membrane, CD81 endocytosis and the CD81-Claudin-1 receptor complex in HCV internalization. Additional functions for CD81 in the viral life cycle and the role of HCV-CD81 interactions in HCV-induced B-cell and CNS (central nervous system) abnormalities are discussed.

Hepatitis B and C virus hepatocarcinogenesis: lessons learned and future challenges

Worldwide, hepatocellular carcinoma (HCC) is one of the most common cancers. It is thought that 80% of hepatocellular carcinomas are linked to chronic infections with the hepatitis B (HBV) or hepatitis C (HCV) viruses. Chronic HBV and HCV infections can alter hepatocyte physiology in similar ways and may utilize similar mechanisms to influence the development of HCC. There has been significant progress towards understanding the molecular biology of HBV and HCV and identifying the cellular signal transduction pathways that are altered by HBV and HCV infections. Although the precise molecular mechanisms that link HBV and HCV infections to the development of HCC are not entirely understood, there is considerable evidence that both inflammatory responses to infections with these viruses, and associated destruction and regeneration of hepatocytes, as well as activities of HBV- or HCV-encoded proteins, contribute to hepatocyte transformation. In this review, we summarize progress in defining mechanisms that may link HBV and HCV infections to the development of HCC, discuss the challenges of directly defining the processes that underlie HBV- and HCV-associated HCC, and describe areas that remain to be explored.

Antiviral drugs against hepatitis C virus

Hepatitis C virus (HCV) infection is a major worldwide problem causes acute and chronic HCV infection. Current treatment of HCV includes pegylated interferon-α (PEG IFN- α) plus ribavirin (RBV) which has significant side effects depending upon the type of genotype. Currently, there is a need to develop antiviral agents, both from synthetic chemistry and Herbal sources. In the last decade, various novel HCV replication, helicase and entry inhibitors have been synthesized and some of which have been entered in different phases of clinical trials. Successful results have been acquired by executing combinational therapy of compounds with standard regime in different HCV replicons. Even though, diverse groups of compounds have been described as antiviral targets against HCV via Specifically Targeted Antiviral Therapy for hepatitis C (STAT-C) approach (in which compounds are designed to directly block HCV or host proteins concerned in HCV replication), still there is a need to improve the properties of existing antiviral compounds. In this review, we sum up potent antiviral compounds against entry, unwinding and replication of HCV and discussed their activity in combination with standard therapy. Conclusively, further innovative research on chemical compounds will lead to consistent standard therapy with fewer side effects.

Comprehensive linker-scanning mutagenesis of the hepatitis C virus E1 and E2 envelope glycoproteins reveals new structure-function relationships

Despite extensive research, many details about the structure and functions of hepatitis C virus (HCV) glycoproteins E1 and E2 are not fully understood, and their crystal structure remains to be determined. We applied linker-scanning mutagenesis to generate a panel of 34 mutants, each containing an insertion of 5 aa at a random position within the E1E2 sequence. The mutated glycoproteins were analysed by using a range of assays to identify regions critical for maintaining protein conformation, E1E2 complex assembly, CD81 receptor binding, membrane fusion and infectivity. The results, while supporting previously published data, provide several interesting new findings. Firstly, insertion at amino acid 587 or 596 reduced E1E2 heterodimerization without affecting reactivity with some conformation-sensitive mAbs or with CD81, thus implicating these residues in glycoprotein assembly. Secondly, insertions within a conserved region of E2, between amino acid residues 611 and 631, severely disrupted protein conformation and abrogated binding of all conformation-sensitive antibodies, suggesting that the structural integrity of this region is critical for the correct folding of E2. Thirdly, an insertion at Leu-682 specifically affected membrane fusion, providing direct evidence that the membrane-proximal ‘stem’ of E2 is involved in the fusion mechanism. Overall, our results show that the HCV glycoproteins generally do not tolerate insertions and that there are a very limited number of sites that can be changed without dramatic loss of function. Nevertheless, we identified two E2 insertion mutants, at amino acid residues 408 and 577, that were infectious in the murine leukemia virus-based HCV pseudoparticle system.

Unique ties between hepatitis C virus replication and intracellular lipids

Hepatitis C virus (HCV) infects approximately 3% of the world's population, establishing a lifelong infection in the majority of cases. The life cycle of HCV is closely tied to the lipid metabolism of liver cells, and lipid droplets have emerged as crucial intracellular organelles that support persistent propagation of viral infection. In this review, we examine recent advances in our understanding of how HCV usurps intracellular lipids to propagate, and highlight unique opportunities for therapeutic intervention.

Current drug discovery strategies for treatment of hepatitis C virus infection

OBJECTIVES

Hepatitis C virus (HCV) infection represents a major worldwide-health problem. The current standard of care is combination therapy with pegylated interferon and ribavirin, which achieves a successful response in only approximately 40% of genotype I patients.

KEY FINDINGS

The biology of HCV infection has been under intensive research and important progress has been made in understanding the replication cycle of the virus. Several therapeutic targets have been under investigation, such as NS3 protease, NS4A replicase and NS5B polymerase. New potential targets, such as NS2 protease, as well as CD-81 and claudin-1 entry co-receptors, have also been identified.

SUMMARY

Clinical evaluations of drug candidates targeting NS3 protease, NS4A cofactor, and NS5B polymerase have demonstrated the potential of developing small molecules that interfere with the replication of the virus. Additional issues, including genotype coverage, resistant mutations, and combination therapy represent major challenges for future drug discovery efforts.

EGFR and EphA2 are host factors for hepatitis C virus entry and possible targets for antiviral therapy

Hepatitis C virus (HCV) is a major cause of liver disease, but therapeutic options are limited and there are no prevention strategies. Viral entry is the first step of infection and requires the cooperative interaction of several host cell factors. Using a functional RNAi kinase screen, we identified epidermal growth factor receptor and ephrin receptor A2 as host cofactors for HCV entry. Blocking receptor kinase activity by approved inhibitors broadly impaired infection by all major HCV genotypes and viral escape variants in cell culture and in a human liver chimeric mouse model in vivo. The identified receptor tyrosine kinases (RTKs) mediate HCV entry by regulating CD81-claudin-1 co-receptor associations and viral glycoprotein-dependent membrane fusion. These results identify RTKs as previously unknown HCV entry cofactors and show that tyrosine kinase inhibitors have substantial antiviral activity. Inhibition of RTK function may constitute a new approach for prevention and treatment of HCV infection.

Spatial regulation of Dia and Myosin-II by RhoGEF2 controls initiation of E-cadherin endocytosis during epithelial morphogenesis

E-cadherin plays a pivotal role in epithelial morphogenesis. It controls the intercellular adhesion required for tissue cohesion and anchors the actomyosin-driven tension needed to change cell shape. In the early Drosophila embryo, Myosin-II (Myo-II) controls the planar polarized remodelling of cell junctions and tissue extension. The E-cadherin distribution is also planar polarized and complementary to the Myosin-II distribution. Here we show that E-cadherin polarity is controlled by the polarized regulation of clathrin- and dynamin-mediated endocytosis. Blocking E-cadherin endocytosis resulted in cell intercalation defects. We delineate a pathway that controls the initiation of E-cadherin endocytosis through the regulation of AP2 and clathrin coat recruitment by E-cadherin. This requires the concerted action of the formin Diaphanous (Dia) and Myosin-II. Their activity is controlled by the guanine exchange factor RhoGEF2, which is planar polarized and absent in non-intercalating regions. Finally, we provide evidence that Dia and Myo-II control the initiation of E-cadherin endocytosis by regulating the lateral clustering of E-cadherin.

Scavenger receptor class B type I and the hypervariable region-1 of hepatitis C virus in cell entry and neutralisation

Hepatitis C virus (HCV) infection is a leading cause of chronic liver disease worldwide and represents a major public health problem. Viral attachment and entry - the first encounter of the virus with the host cell - are major targets of neutralising immune responses. Thus, a detailed understanding of the HCV entry process offers interesting opportunities for the development of novel therapeutic strategies. Different cellular or soluble host factors mediate HCV entry, and considerable progress has been made in recent years to decipher how they induce HCV attachment, internalisation and membrane fusion. Among these factors, the scavenger receptor class B type I (SR-BI/SCARB1) is essential for HCV replication in vitro, through its interaction with the HCV E1E2 surface glycoproteins and, more particularly, the HVR1 segment located in the E2 protein. SR-BI is an interesting receptor because HCV, whose replication cycle intersects with lipoprotein metabolism, seems to exploit some aspects of its physiological functions, such as cholesterol transfer from high-density lipoprotein (HDL), during cell entry. SR-BI is also involved in neutralisation attenuation and therefore could be an important target for therapeutic intervention. Recent results suggest that it should be possible to identify inhibitors of the interaction of HCV with SR-BI that do not impair its important physiological properties, as discussed in this review.